Leftover helium that keeps the detector signal raised after a large leak or a heavy spray, because helium has soaked into O-rings, dead volumes and the detector's own plumbing. Until the background falls, smaller leaks cannot be seen. Techs avoid it by spraying sparingly, purging with nitrogen and waiting; a detector's zero or background-suppression function hides the offset but does not restore sensitivity.
We got a 1E-5 hit on the gate valve bonnet and now there's hang-up everywhere, give it twenty minutes.
Helium soaks through a Viton seal within minutes, so a long spray on an O-ring joint can give a slow-rising signal that is permeation, not a leak.
Also heard as
- helium hang-up
Related terms
Helium leak detector
Mass spectrometer tuned to helium that finds and measures leaks by detecting helium drawn into the system.
Helium spray test
Outside-in leak test: helium is sprayed on an evacuated part while a detector watches for it inside.
O-ring
Elastomer ring with a round cross-section that seals demountable vacuum joints by being squeezed into place.
Permeation
Gas passing through the solid walls or seals of a vacuum system by dissolving into and diffusing through them.